1. Field of the Invention
The present invention relates generally to gas flow rate calibration and in particular to flow rate calibration systems for accurately measuring and establishing gas flow rates for flow meters and similar devices.
2. Description of the Prior Art
Flow rate measurement devices are used in conjunction with many industrial and scientific processes in order to measure, sense, and control gas flow rates. Such devices include flow meters, flow sensors, and flow controllers. To ensure the accuracy of a flow meter or similar device over a given range of flow rates, calibration methods capable of operating over the same range of flow rates must be available and operate accurately.
A well known method for calibrating flow rate measurement devices is the piston-tube prover method. Generally, in the piston-tube prover method, gas or other fluid is directed through the device to be calibrated which measures the flow rate in the normal manner. The gas exits the device to be calibrated and is directed to the bottom of a precision ground glass tube of a known internal diameter. A piston installed inside the tube rises according to gas flow into the bottom of the tube. After a known time interval, the displacement of the piston is measured and is multiplied by the cross-sectional internal area of the tube to determine the volume of gas that has entered the tube. This volume divided by the time interval (with adjustments for gas temperature and pressure) equals the actual gas flow rate from the flow device. The actual flow rate as measured by the piston displacement can be compared to the flow rate measured by the flow device, and the flow device can be calibrated accordingly.
The piston-glass tube prover method normally employs a light weight piston with a seal that creates very little friction against the inner surface of the glass tube. Tests have shown that the piston must be able to rise up the glass tube with a pressure differential from the bottom to the top of the piston not in excess of five inches of water in order that very precise gas flow rate measurements may be made. The piston""s low friction seal also must be leak proof to maintain high accuracy.
So far as is known, previous piston provers utilized a mercury, a known hazardous fluid, seal to ensure low friction and zero gas leakage past the seal. For example, the piston-tube provers in U.S. Pat. No. 3,125,879 and U.S. Pat. No. 5,526,674 used a mercury ring seal around the piston. When piston provers were originally designed, mercury was considered to be a less dangerous material, and manufacturers were not aware of potential liability associated with the sale of mercury. However, over time the dangers and liabilities associated with mercury became well known and operator safety became a paramount concern. Various attempts were made by gas calibration equipment manufacturers to limit their liability when selling piston provers with mercury seals. For example, manufacturers provided documentation explaining the dangers of mercury, placed hazardous material labels on the piston provers to alert operators of possible health risks, and added special filters to capture mercury vapors exiting the glass tubes.
Numerous types of pistons are well known outside of the art of piston provers, but so far as is known none of these pistons were designed for use in a piston-tube prover and none are suitable for such a use. For instance, the piston disclosed in U.S. Pat. No. 3,994,208 used a piston which was unsuitable as a prover. It was designed for a high pressure environment, was constructed of a heavy, stiff material, and relied on a connecting rod to maintain its alignment within a cylinder. The piston had a cylindrical body with a deformable cylindrical flange at one end, and at rest the outside diameter of the flange was less than the inside diameter of the cylinder into which it was installed. In operation, the flange would not deform during the low pressure piston strokes, thus maintaining a relatively large clearance between the piston and the cylinder. During the high pressure strokes, where the pressure on the top of the piston was much greater than the pressure on the bottom of the piston, the flange would deform and press against the cylinder wall to form a tight seal with the cylinder. As a result, the piston maintained a tight seal against the cylinder wall only when the flange was subjected to very high pressure differential across the deformable flange, thereby pressing the flange tightly against the cylinder wall, which was the intended result of the design. However, this flange design was unsuitable for a piston-tube prover because it relied on a high pressure differential to create an effective seal against the cylinder wall, and the seal was only maintained part of the time. Without a high difference in pressure across the flange, the flange would not deform and press against the cylinder wall, and would thus allow gas leakage past the piston. An accurate piston-tube prover for gas flow requires a piston that provides an effective seal even when the pressure difference from the top of the piston to the bottom of the piston is as low as five inches of water or less.
U.S. Pat. No. 5,884,550 disclosed a piston for an internal combustion engine or compressor. The piston of this patent included a single cantilevered seal around its circumference which was designed to replace a traditional piston ring. The seal had a slightly larger diameter than the piston body (approximately 0.001 to 0.003 inches), and the seal had a uniform thickness from base to tip. To create a tighter seal, the seal was manufactured with a higher coefficient of thermal expansion and/or lower mechanical modulus than the rest of the piston. During operation, the high pressure from the engine or compressor exerted force on the seal causing it to press against and maintain contact with the cylinder. As a result, the piston relied on high pressure to create an effective seal against the cylinder wall. The integrity of the seal was created by forcing the seal against the side of the cylinder wall. The pressure differential across the seal was high enough to overcome any friction between the seal and the wall. Also, the piston was constructed of carbon or carbon composite to reduce weight and improve heat dissipation. Such a material would not be capable of flexing and conforming closely to the wall of the cylinder without the high pressure seal against the wall. If any non-uniformities existed in the cylinder wall, the piston would catch and possibly get stuck in the cylinder absent a large pressure differential to push the piston through the cylinder. Finally, the piston relied on a single seal and a connecting rod to maintain the alignment of the piston within the cylinder. The piston was not xe2x80x9cfree floating,xe2x80x9d but was instead guided through the cylinder wall by the connecting rod.
The present invention provides a tube prover flow calibration system. The flow calibration system according to the present invention includes at least one calibration tube which has an inner surface and an inner diameter. A piston with an outer end, an inner end and an outside diameter is disposed within the calibration tube. The piston itself forms a seal with the inner surface of the cylindrical tube and includes at least one flexible sealing skirt. Such a sealing skirt is located toward the outer end of the piston and maintains continuous contact with the inner surface of the tube. A processor computes gas flow rate based upon measurements of the movement of the piston within the tube and other factors.
The present invention also provides a low friction piston-glass tube prover assembly. The assembly according to the present invention includes a calibration tube which has an inner surface. A piston with an outer end and an outside diameter is disposed within the calibration tube and forms a seal with the inner surface of the calibration tube. The piston also includes at least one flexible sealing skirt which is in contact with the inner surface of the calibration tube.
The present invention further provides a low friction piston for calibrating a gas flow meter. The piston according to the present invention includes a piston body. The piston body moves longitudinally within the calibration tube in response to a gas flow in the calibration system. The piston body is also made from a resilient material and includes at least one sealing skirt which extends outwardly from the piston body. The sealing skirt contacts the inner surface of the calibration tube and seals the calibration tube against leakage of the gas flow past the piston body in the calibration tube.